Email: Do not delete the message from server if the user set it as "Never"

Checking the pop3 account's delete policy when delete the mail.
If the user set the delete policy as "never", then we needn't to
delete the mail from server, but only delete the mail from local.

CRs-Fixed: 625820

Change-Id: I77abd62e079e46f0a77961e7c9eab6ca94a59ec2
Email: Notify UI data changed of the unread mailbox.

Notify UI the data has been changed of the unread mailbox and UI will
update the content according to the new data.

CRs-fixed: 630974

Change-Id: I041d2638c8c4c892e6207cdac7dd89edf34d511f